During optimal operation a satellite antenna beam center or pointing position on the earth is kept within a small box in both directions of North-South and East-West around its nominal position. When a beam center position is within this optimal box, the beam coverage on the earth is stable, communications between the earth station and the satellite are stable and earth station antennas do not have to be often repositioned for tracking the satellite to maintain stable communications.
Electronic equipment on board a satellite is designed to function quite a long time. Thus, the determining factor of a satellite's operational lifespan is not electronic equipment life, but rather station-keeping fuel supply. Station-keeping is the process of adjusting a satellite's orbital position such that the satellite's position relative to the earth falls within acceptable parameters. When the station-keeping fuel is exhausted the satellite's useful life comes to an end.
The amount of station-keeping fuel necessary to perform satellite North-South position correction (inclination correction) is much more than that necessary for East-West position correction. Thus, it is a common practice that only East-West position correction is performed when the fuel on board the satellite is close to exhaustion. The lack of North-South position correction forces the satellite into an inclined orbit. Without North-South position correction, the satellite's orbital inclination will increase about 0.9° per year. As an example, Table 1 shows predicted date and inclination values for a satellite positioned at 52.50° E in an inclined orbit.
TABLE 1Inclinations of a Satellite at 50.50° EDatePredicted Inclination (Degrees)Dec. 31, 20020.94Dec. 31, 20031.86Dec. 31, 20042.80Dec. 31, 20053.75Dec. 31, 20064.69Dec. 31, 20086.54
While a satellite is in an inclined orbit, the satellite's position relative to the earth keeps moving. This movement causes degradations in communications performance between the satellite and the earth station. More particularly, relative position movement due to an inclined orbit causes the satellite looking angles (azimuth and elevation) from an earth station to change as the satellite's relative position moves, resulting in changes of the earth station antenna gain toward the satellite. This causes communication performance degradation. The earth station antenna has to keep tracking the satellite, i.e., be repeatedly repositioned to stay in contact with the satellite, in order to solve this problem.
Another problem arising from satellite relative position movement is that the satellite's polarization direction at the earth station changes as the satellite's relative position moves. This also results in the communication performance degradation. The earth station antenna has to keep tracking the satellite's polarization direction in order to solve this problem. However, polarization direction usually does not cause great performance degradation within a reasonable range of inclination.
Satellite relative position movement also causes looking angles (azimuth and elevation) from the satellite toward an earth station to change as the satellite's relative position changes due to the inclined orbit. This results in changes of the satellite antenna's gain toward the earth station, which also contributes to communication performance degradation.
If a satellite has a zero inclination and is in a perfect geo-stationary orbit, the satellite antenna's beam pointing position on the earth will be at the nominal position constantly. However, when a satellite is in inclined orbit, the beam pointing position on the earth will be off from the nominal position and move with time on an ellipse. The shape and size of the ellipse depends on the beam pointing position relative to the satellite sub-point, which is a point on the earth having same latitude and longitude as the satellite, and on the inclination magnitude.
FIG. 1 shows, as an example, a nominal antenna beam pointing position at 37.18° N in latitude and 46.27° E in longitude for a satellite at 52.50° E. Six ellipses of antenna beam pointing around the nominal position are shown in FIG. 1 for inclinations listed in Table 1 (shown above). The movement of the antenna beam pointing position will cause beam coverage on the earth to move accordingly, resulting in a change of the satellite antenna gain toward an earth station, in turn causing communication performance degradation, especially at beam edge. As shown, the larger the inclination, the larger the ellipse. With a larger ellipse comes an increase in communication performance degradation.
FIG. 2 is another view of a satellite's beam center position path 1 around a desired beam center position (shown as Washington D.C.) for a satellite in an inclined orbit. In a particular inclined orbit the beam center is moving around the path 1. Rings A show the satellite antenna gain contours when the satellite is at 12:00 o'clock. Rings B show the satellite antenna gain contours when the satellite is at 6:00 o'clock position. And, rings C show the satellite antenna contours when the satellite is at 0:00 o'clock position.
As shown, at the 0:00 o'clock location of the beam center, the signal received at D.C. has a 5 db loss, with the beam center at the 6 o'clock location, the loss in D.C. is a 4 db, and with the beam center at the 12 o'clock location the loss at Washington D.C. is approximately 3 db. Typically, a loss of 4 or 5 db is unacceptable. That is, a satellite's signal is not usable due to degradation when the satellite is in a position resulting in such a loss.
Turning to FIG. 3 it has been proposed to solve the above-mentioned problems by transmitting a beacon signal 100 from an earth station 110 to a satellite's antenna 120. As shown, satellite antenna 120 has a center target point 122. However, as also shown, the beacon signal 100 is actually centered at point 122A on the face of the satellite antenna 120, and accordingly, an offset exists between the desired location 122 of the beacon signal 100 on the satellite antenna 120 and the actual location 122A of the beacon signal 100 on the satellite antenna 120. After receipt at the satellite, the beacon signal 100 is transmitted to an onboard satellite tracking receiver 130 which detects the beacon signal 100, and based upon signal parameters (magnitude and sign), determines the offset between point 122 and 122A. The satellite tracking receiver 130 then generates a satellite antenna drive signal to correct the azimuth and elevation angles of the satellite antenna 120. This generated antenna drive signal is transmitted to an on-board antenna drive unit 140 which drives the satellite's antenna 120 to correct its azimuth and elevation angles in order to move antenna 120 such that the beacon signal 100 is centered on desired point 122, thereby eliminating the offset in the beacon signal 100 receiving location and the desired beacon signal 100 receiving location at the antenna 120.
This proposed solution requires substantial additional hardware to be installed on-board the satellite which adds weight and complexity to the satellite and also potentially decreases reliability. However, an even greater problem arises due to the fact that the satellite antenna 120 will necessarily be redirected during the life of the satellite to move the spot beam to different locations on the earth's surface from time to time. For example, referring back to FIG. 2, there may be a time when it will be desirable to have the spot beam centered on Chicago rather than Washington D.C. Accordingly, the earth station 110 from which the beacon signal 100 is transmitted would need to move each time that the spot beam was moved. Such movement of the beacon station makes the proposed solution impractical and virtually impossible to implement successfully.
Accordingly, a need remains for a technique which does not require a beacon signal to maintain a position of an antenna footprint at the earth's surface at a desired location when the satellite is in an inclined orbit.